The invention generally relates to a power-supply apparatus for starting and operating high-pressure gas discharge lamps, which is fed from a battery, and which has at least one current- or power-controlled voltage converter coupled to a high-pressure gas discharge lamp via an ignition device and to a current/power control device via an input terminal, to which is applied an input signal corresponding to lamp voltage, and an output terminal through which a current/power nominal, or desired, value is supplied to the voltage converter; wherein the current/power control device comprises a comparator which produces a switching signal dependent upon the input signal and a capacitor which, dependent upon the produced switching signal, is charged with a first time constant or discharged with a second time constant and whose voltage determines the level and duration of an additional voltage supplied after ignition of the high-pressure gas discharge lamp, which in turn depends upon an operational status of the high-pressure gas discharge lamp and is limited by its allowable maximum lamp current.
From German patent application no. P 40 15 398, which is not prior art, a power-supply apparatus for starting and operating high-pressure gas discharge lamps is known wherein the power-supply apparatus is fed from a battery of a motor vehicle. The known power-supply apparatus has a current- and power-controlled voltage converter which is an ac-converter to produce an ac-voltage from a dc-voltage applied from the vehicle battery. The voltage converter is coupled to a high-pressure gas discharge lamp via an ignition device. The voltage converter is coupled to a current/power control device for regulating the allowable maximum current and power supplied to the high-pressure gas discharge lamp. Also, the current/power control device has an output terminal through which a current/power nominal, or desired, value is fed to the voltage converter. In order for the high-pressure gas discharge lamp to reliably reach an active operational status as quickly as possible after ignition, the current/power control device has a circuit arrangement by means of which the high-pressure gas discharge lamp is supplied, immediately after ignition, with additional power from the voltage converter, dependent upon a respective operational status of the high-pressure gas discharge lamp and limited by its allowable maximum lamp current. In this respect, the known circuit arrangement has a comparator, which produces a switching signal dependent upon the charging state of a first capacitor, and a second capacitor, which, dependent upon a produced switching signal, is charged at a first time constant or discharged at a second time constant, wherein the voltage of the capacitor determines the level and duration of the additional power supplied to the high-pressure gas discharge lamp. The first capacitor is charged and also discharged by means of a third time constant which determines the time period during which the second capacitor is not discharged.
With the known power-supply apparatus for ignition and operation of high-pressure gas discharge lamps, it has proven to be of disadvantage that the circuit arrangement of the current/power control device, which determines the duration and level of the additional power supplied to the high-pressure gas discharge lamp, is constructed in a cost-intensive manner thereby causing high costs in overall manufacture. In the known circuit arrangement of the power-supply apparatus, two capacitors are coupled with resistors to have various time constants for their charging and discharging and three operational amplifiers and two circuit arrangements are used for the determination of additional power. Also, it proves to be of disadvantage that in the known current/power control device a status-determination device is required which determines whether the high-pressure gas discharge lamp is operating. It has been known to be particularly disadvantageous in the known current/power control device that the first capacitor, which determines the level and duration of the additional voltage to be supplied to the high-pressure gas discharge lamp immediately after ignition, is discharged if the power-supply apparatus is not activated and is charged if the power-supply apparatus is activated so that upon the occurrence of residual, or leakage, currents of the first capacitor, the first capacitor cannot reach a desired charging state during steady operation, thereby not maintaining lamp nominal power.
It proves to be especially disadvantageous that the point in time of reducing additional power is determined by a lamp-independent time constant.
It is an object of this invention to provide a power-supply apparatus which can be manufactured uncomplicatedly and cost-effectively and has a power-control device which ensures optimal ignition and reliable steady operation of a lamp.